(1) Field of the Invention
This invention relates generally to transducer arrays that are towed by ocean vessels. More particularly, this invention relates to towed arrays having auxiliary transducers useful to determine the direction of origin of a sonar signal impinging on the array.
(2) Description of the Prior Art
Arrays of hydrophones are conventionally towed by surface ships and submarines for the purpose of sensing sound below the surface of the ocean. Typically, such arrays are linear assemblies of hydrophones mounted in an elongated plastic or rubber hose. The hose sections are filled with a fluid that surrounds the hydrophones. Sound pressure waves in the ocean pass through the hose wall and into the fluid that surrounds the hydrophones. The hydrophones sense the pressure fluctuations and transforms the sensed pressures into electrical signals which are transmitted by a cable back to the vessel. The electrical signals are then processed to derive characteristics of the sound or a representation thereof.
A well-known problem with conventional towed arrays is that when beamforming electronics are used to determine the incident angle of the sonar signal, it is generally not possible to determine whether the sonar source is positioned to the left or the right of the array. This situation is usually referred to as the "left-right ambiguity problem."
One known way of dealing with the left-right ambiguity problem is to take a first measurement, maneuver the towship to another position, and take a second measurement in order to break the symmetry. However, this technique requires a considerable amount of time, because after the towship is turned, the array can take up to twenty minutes to straighten. Another known technique is to introduce a known distortion into the shape of the array, and then to correct the beamformer to compensate for this distortion. However, this method has a drawback in that the degree of distortion must be accurately determined.